Water adsorption on kaolinite, illite, and montmorillonite clays was studied as a function of relative humidity (RH) at room temperature (298 K) using horizontal attenuated total reflectance (HATR) Fourier transform infrared (FTIR) spectroscopy equipped with a flow cell. The water content as a function of RH was modeled using the Brunauer, Emmett, and Teller (BET) and Freundlich adsorption isotherm models to provide complementary multilayer adsorption analysis of water uptake on the clays. A detailed analysis of model fit integrity is reported. From the BET fit to the experimental data, the water content on each of the three clays at monolayer (ML) water coverage was determined and found to agree with previously reported gravimetric data. However, BET analysis failed to adequately describe adsorption phenomena at RH values greater than 80%, 50%, and 70% RH for kaolinite, illite, and montmorillonite clays, respectively. The Freundlich adsorption model was found to fit the data well over the entire range of RH values studied and revealed two distinct water adsorption regimes. Data obtained from the Freundlich model showed that montmorillonite has the highest water adsorption strength and highest adsorption capacity at RH values greater than 19% (i.e., above ML water adsorption) relative to the kaolinite and illite clays. The difference in the observed water adsorption behavior between the three clays was attributed to different water uptake mechanisms based on a distribution of available adsorption sites. It is suggested that different properties drive water adsorption under different adsorption regimes resulting in the broad variability of water uptake mechanisms.

Copper nanoparticles exhibit intense and sharp localized surface plasmon resonance (LSPR) in the visible region; however, the LSPR peaks become weak and broad when exposed to air due to the oxidation of Cu. In this work, the Cu nanoparticles are successfully encapsulated in SiO 2 by employing trioctyl-n-phosphine (TOP) capped Cu nanoparticles for the sol-gel reaction, yielding an aqueous Cu-SiO 2 core-shell suspension with stable and well-preserved LSPR properties of the Cu cores. With the TOP capping, the oxidation of the Cu cores in the microemulsion was significantly reduced, thus allowing the Cu cores to sustain the sol-gel process used for coating the SiO 2 protection layer. It was found that the self-assembled TOP-capped Cu nanoparticles were spontaneously disassembled during the sol-gel reaction, thus recovering the LSPR of individual particles. During the disassembling progress, the extinction spectrum of the nanocube agglomerates evolved from a broad extinction profile to a narrow and sharp peak. For a mixture of nanocubes and nanorods, the spectra evolved to two distinct peaks during the dissembling process. The observed spectra match well with the numerical simulations. These Cu-SiO 2 coreshell nanoparticles with sharp and stable LSPR may greatly expand the utilization of Cu nanoparticles in aqueous environments.

Controlling the deposition of exotic metals in the seeded growth of multimetal nanostructures is challenging. This work describes a seeded growth method assisted by a mask for synthesis of segmented binary or ternary metal nanostructures. Silica is used as a mask to partially block the surface of a seed and a second metal is subsequently deposited on the exposed area, forming a bimetallic heterodimer. The initial demonstration was carried out on a Au seed, followed by deposition of Pd or Pt on the seed. It was found that Pd tended to spread out laterally on the seed while Pt inclined to grow vertically into branched topology on Au. Without removal of the SiO 2 mask, Pt could be further deposited on the unblocked Pd of the Pd−Au dimer to form a Pt−Pd−Au trimer. Mask-assisted seeded growth provides a general strategy to construct segmented metallic nanoarchitectures. ■ INTRODUCTIONHeterostructures of two or more metals with interfaces at the nanoscale is of particular significance because they exhibit unique properties and multifunctions distinctly different from the individual components. 1−4 The diverse surface chemistry of these heteronanostructures enables new applications that are not possible with each component alone. For example, multisegmented metal nanorods have been demonstrated for applications in synergistic heterogeneous catalysis, 5,6 selfelectrophoretic nanomotors/nanobatteries, 7,8 multifunctional biomedicine, 9,10 and multiplexed detections. 11,12 To fabricate the heteronanostructures, sequential electrochemical deposition of metal ions into templates has been the most common method since the 1990s. 13,14 In this approach, commercially available alumina or polycarbonate membranes with uniform pores are often used as templates, yielding segmented metal rods. 15 This method could be further extended to selective growth of additional metals within the templates in solution after the initial electrochemical deposition of rod-shaped seeds. 16,17 To generate heterostructures with other configurations, cost-intensive and time-consuming lithography techniques are usually required. 18 In this work, a site-selective seeded growth method, termed mask-assisted seeded growth (MASG), is developed to expand the library of metal heterostructures with complex nanoarchitectures.Seeded growth has emerged as a compelling method to create a wide variety of novel metal nanostructures. 19−24 Conditions that yield heteronanostructures depend on a number of factors such as the structural characteristics of constituent components, the reduction kinetics of metal precursors, and the capping agents. For example, a high degree of lattice mismatch between the seed and the second metal prevents conformal growth of core@shell structures and yields heteronanostructures of Au on CoPt 3 , 25 Au rods on Pt cubes, 21 and Cu on Au. 24 Controlling the reduction kinetics can selectively direct the nucleation and subsequent growth of the second metal on the seed to form dimers of Au on Pd 26 or other nonconformal structures such as Ag/...

In this work, the impact of structure and composition on the dealloying of bulk and nanoscale alloys Cu x Au(1–x) have been discussed. In comparison with the dealloying of AgAu alloys, the CuAu system exhibits dealloying curves with more features associated generally with multistage dealloying. It has been shown for the first time that three stages exist during dealloying process of bulk Cu x Au(1–x) (x = 0.7 and 0.8) alloys. The dealloying critical potential, E c, has been associated with the starting point of stage II in which the anodic current slowly increases. Analysis of data from this work along with results of others suggests a monotonic potential dependence of E c upon the composition of bulk Cu x Au(1–x) alloys in the range of x from 0.70 to 0.95. The dealloying behavior of Cu0.75Au0.25 (Cu3Au) intermetallic (length ∼19 nm, width ∼10 nm) and random alloy (length ∼23 nm, width ∼9 nm) nanorods have also been discussed. Very close values of E c have been determined for both types of nanorods with the random alloy dealloying at slightly more negative potentials (c.a. 15–20 mV) than the intermetallic. In addition, both Cu3Au nanorods feature close to 200 mV lower E c than bulk alloys with identical composition. Formic acid oxidation tests reveal that the catalysts generated by platinization of as-synthesized and dealloyed nanorods exhibit very good activity with peak current densities in the range of 3.5 to 5.5 mA.cm–2. Both catalysts withstand testing of more than 1500 cycles. Overall, the results of this study demonstrate unique aspects of Cu x Au(1–x) dealloying and ascertain the feasibility of nanosized frameworks (dealloyed structures or nanoparticles) as catalyst supports in fuel cell applications.